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Variable air handlers

I'm a homeowner looking to replace my old AC system, and I'm getting conflicting information from different contractors on the benefit of installing a variable speed air handler. One contractor says it saves on energy because of the fact that it can draw more moisture out of the air. The other contractor says you shouldn't install one, because without installing more supply and possibly another return line (according to the contractor something you can't know until a month or two after its installed) it won't pull enough air across the coil to function properly? I appreciate any and all help I can get.

I'm a homeowner looking to replace my old AC system, and I'm getting conflicting information from different contractors on the benefit of installing a variable speed air handler. One contractor says it saves on energy because of the fact that it can draw more moisture out of the air. The other contractor says you shouldn't install one, because without installing more supply and possibly another return line (according to the contractor something you can't know until a month or two after its installed) it won't pull enough air across the coil to function properly? I appreciate any and all help I can get.

IMHO,they are very efficient
sounds to me,that dipstick can't set a dipswitch

The government works for me. I do not answer to them, they answer to me.

we need a few more sheep dogs to keep barking at the wolves,and the stupid sheep

A well-regulated militia, being necessary to the security of a free State, the right of the people to keep and bear arms, shall not be infringed.

I'm a homeowner looking to replace my old AC system, and I'm getting conflicting information from different contractors on the benefit of installing a variable speed air handler. One contractor says it saves on energy because of the fact that it can draw more moisture out of the air.

If properly set up, this is true. Either using an algorithm, or a (de)humidistat, a variable speed system can run at lower fan speed part of the time in cooling mode, which will pull humidity out faster.

Originally Posted by BilltheCat

The other contractor says you shouldn't install one, because without installing more supply and possibly another return line (according to the contractor something you can't know until a month or two after its installed) it won't pull enough air across the coil to function properly? I appreciate any and all help I can get.

That's funny, because some have come here after being told that a VS system was the solution to their undersized ducts problem. It's not.

A contractor that knows what he is doing can tell you whether your duct system is good enough, either by doing sizing according to "Manual D" or actual airflow measurements on your old system.

Variables are definately not a cure all, for poor duct design, but there should be no reason, that it cannot be installed, The manual d is to evaluate as to how much this is going to help your airflow.

Last edited by aintitfun; 07-13-2008 at 09:35 PM.
Reason: spelled like a kindergartner

Thanks for your help so far, I feel like at least I have somewhere to turn to. Understand, I am not trying to DYI, but its not like I just have $6000 lying around to plunk down on the first whiz bang system someone tries to sell me. I've got three different contractors that I've contacted for quotes. None has so far done the Manual J calc, though when I questioned the first contractor about it he said that since nothing had changed in the house since it was built (16 years ago).

I want a good reliable system that will not only cool my house, but will be energy efficient and reliable as well. Since I can't rely on Consumer Reports or any independent source like that, I'm having to try and figure this out myself. Thanks for your time.

Easier to just test the static of the existing system first,then Man. to redesign if needed.

This is sound advice. Even if the ducting system is not perfect, which most are not, an ECM motor can be beneficial as long as the current static pressure parameters are met.

For example; if your system is a 2.5 ton system, the nominal amount of air needed, based on the average 400 cfm per ton, would be 1000 cfm. Let's say your duct system calculates to be between .7 and .8 SP, which is not far from what I've seen nationwide. This would put the system at the far end of the blower charts for a standard psc motor but still within the capabilities of a VS motor.

If the blower chart for a psc motor blower rate the required 1000 cfm of air at the standard SP of .5, then at .7-.8 SP that psc blower will only be providing about 700-800 cfm of air. With a psc motor, this can be a problem during humid days when a wet coil from low airflow will create even more resistance which will lower the cfm ability even more until the coil eventually freezes up.

With the same system using a VS blower, the VS motor set at 1000 cfm is going to continually ramp up to provide that 1000 cfm of air. There is no SP diferential chart for a VS blower because the motor continues to attempt to achieve the amount of air it is programmed despite varying SP conditions...within reason.

Also, a VS blower can be safely set to a lower cfm rating without the fear of the additional resistance from a wet coil causing a coil freeze up. By setting a VS blower to a lower cfm rating, you are decreasing the SP of the system, creating an all around quieter and better dehumidifying system.

If a duct system is just too restrictive, a VS motor will do what is called "huffing". This is where the motor ramps up to achieve the desired cfm of air but reaches a critical point where the motor cannot safely operate, so the motor decreases the rpm in order to operate within a safe speed, and will continue going up and down until if eventually dies. In this case, a redesign of the ducting to reduce resistance is needed.

If the ducting can not be practically altered to decrease the SP, then the next best thing to do is to have a larger HP psc motor installed at the same rpm in order to overcome the SP. This will void any motor warranty of a unit and will cost more to operate, so it is only suggested is all else is just not practical.